Field of the Invention
The present invention relates to a combustion chamber including a diverging portion.
In the description below, the terms “upstream” and “downstream” are defined relative to the normal flow direction of fluid in the combustion chamber along the walls of said chamber. The terms “inner” and “outer” indicate a region situated at (or facing towards) respectively the inside and the outside of the combustion chamber.
More particularly, the invention relates to a combustion chamber extending along a longitudinal axis and including a fluid injection system from which there extends in a downstream direction a wall presenting a throat and a diverging portion situated downstream from the throat.
Description of the Related Art
Consideration is given in particular to a rocket engine combustion chamber extending in a longitudinal direction defined by its axis of symmetry, the combustion chamber thus being substantially axisymmetric. The axis of symmetry is thus contained with the combustion chamber, unlike combustion chambers that are annular. In such combustion chambers, the propellant components (fuel and oxidizer, e.g. liquid hydrogen and liquid oxygen) are injected into one end 11 of the chamber 10 by injectors of the injection system. FIG. 1 shows such a combustion chamber. The combustion reaction between the propellant components produces combustion gas (e.g. steam) that is expelled via a throat 15 situated opposite from the injectors. Downstream from the throat 15 (the location where the section of the combustion chamber is the smallest), the chamber flares in a diverging portion 20, which serves to increase the speed of the combustion gas expelled through the throat 15, and thus to increase the thrust delivered by the engine. The diverging portion 20 of the chamber 10 is extended downstream by a diverging portion or “bell” 80 of the rocket engine. This engine bell 80 is fastened to the downstream end 25 of the diverging portion 20 of the chamber 10, and it is a portion of the rocket engine that is distinct from the combustion chamber 10.
While the rocket engine is in operation, the walls of the combustion chamber 10, including the wall 30 of the diverging portion 20 of the chamber 10, are raised to very high temperatures (the combustion gas may be at a temperature of about 3500 kelvins (K) upstream from the throat 15 when combustion is between oxygen and hydrogen) and they need to be cooled (their temperature at the throat must not exceed 1000 K) in order to conserve their mechanical properties. The most usual method for performing such cooling consists in causing one of the propellant components to flow in or in contact with the wall 30 of the diverging portion 20 of the chamber 10 since these components are at a very low temperature (e.g. they are liquefied gases). This flow may take place for example in tubes, either embedded in the wall 30 or else covering the wall 30. Alternatively, this flow may take place in open channels that are formed in the wall 30 or else that are fitted to the radially outer face of the wall 30, these channels being closed by a deposit or by a shell.
While the engine is in operation, the high-speed expulsion of the gas generates very large forces on the engine bell 80, which forces are transferred to the wall 30 as longitudinal forces acting on the downstream end of the combustion chamber 10. Furthermore, the aerodynamic forces encountered in flight, e.g. together with the steering forces applied to the engine by actuators acting on the chamber 10, or forces transmitted by other stationary members on the chamber 10, give rise to forces that are both transverse and longitudinal, in particular at the downstream end of the combustion chamber 10, thereby leading to high stresses on the chamber 10, and in particular on the throat 15.
The wall 30 is also subjected to radial forces due to the combustion of the gases inside the chamber 10. In order to avoid the chamber 10 rupturing, it is necessary to reinforce the chamber 10 by increasing the thickness of its walls.
Nevertheless, such an increase in thickness has the effect of preventing thermal expansion of the hot portion of the wall 30 and thus of reducing its lifetime, unless the device for cooling the chamber 10 is modified so as to cool the wall 30 more quickly.
That technique also gives rise to a large increase in the weight of the engine 10, which is harmful, in particular for a rocket engine, which needs to present a weight that is as small as possible.